Long-Term Evolution (LTE) is the next step in 3rd Generation (3G) cellular networks, which represents basically an evolution of the current mobile communications standards. LTE is considered by many to be a Fourth Generation (4G) technology, both because it is faster than 3G, and because, like the Internet, LTE uses a flat “all-IP” architecture where all information, including voice, is handled as data. LTE provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink, uses scalable bandwidth from 1.25 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations and is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth.
LTE physical layer frame structure is described in the standard 3GPP TS 36.211 “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation” for both FDD mode and TDD mode duplexing procedures. The frame is 10 milliseconds long and is divided in 10 subframes, and every subframe is divided in two slots. The LTE frame transports a set of physical channels and physical signals (also described in said standard). Among these physical signals are the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS), which are used by an LTE User Equipment, UE, to synchronize with the base station (called in LTE evolved or enhanced Node B, eNB) frame and to derive the identity of the cell. Both PSS and SSS are transmitted twice per frame, in the symbols #5 and #6 of slot #0, and in the symbols #5 and #6 of slot #10.
Every slot is divided in 6 or 7 OFDM (Orthogonal Frequency Division Multiplexing) symbols, and every OFDM symbol consists of an initial cyclic prefix and a data part. The cyclic prefix is used to avoid inter-symbol interference in the presence of multi-path propagation. The length of the cyclic prefix of the first symbol of every slot is different to the length of the cyclic prefix of the remaining six symbols, and is equal to 160×(0.5×10−2/153600)=5.2 us in the case of the normal cyclic prefix, and equal to 512×(0.5×10−2/153600)=16.7 us in the case of the extended cyclic prefix.
The PSS is constructed from a sequence of length 63, with the middle element punctured to avoid transmitting on the direct current subcarrier. Three PSS sequences are used in LTE, corresponding to three possible cell identities. The user equipment, UE, (also called mobile terminal) must detect the PSS without any a priori knowledge of the channel, so a non-coherent demodulation must be done. The PSS is used by the UE to determine the timing and the frequency of the LTE signal, and to determine the cell identity. The SSS is constructed from maximum length sequences, and there are 168 possible sequences, corresponding to 168 possible cell identity-groups. Every sequence can be based on any of two codes, which are alternated between the first and second SSS transmissions in each radio frame. This enables the UE to determine the 10 ms radio frame start from a single observation of an SSS.
In OFDM, the controllable radio resource has three aspects: frequency, time and space. A Resource Block (RB) is the basic time-frequency resource allocable for data transmission. Both the PSS and the SSS make use of the six central Resource Blocks (RB) of the LTE radio frame. This six central RB's comprise 72 subcarriers, but the PSS and the SSS only make use of the central 62 subcarriers, in order to demodulate the PSS and the SSS by means of a 64-long FFT (Fast Fourier Transform).
When there is a need to deliver downlink data to a UE in idle mode, the LTE network sends a paging message to all the eNB's in its current Tracking Area (TA), and the eNB's page the UE over the radio interface. On receipt of a paging message, the UE performs a service request procedure which results in moving the UE to the connected state. Paging messages are transported in the Physical Downlink Shared Channel (PDSCH). The Physical Downlink Control Channel (PDCCH) signals a paging indicator, with the detailed paging information being carried on the PDSCH in the Resource Blocks indicated by the PDCCH. Paging indicators on the PDCCH use a single fixed identifier called the Paging Radio Network Temporary Identifier (P-RNTI). Rather than providing different paging identifiers for different groups of UE's, different UE's monitor different subframes for their paging messages.
Another use of the PDSCH is for broadcasting system information blocks (SIB's), which carry System Information (SI) that is not carried on the Physical Broadcast Channel (PBCH). The RB's used for broadcast data in the PDSCH are indicated by signalling messages on the PDCCH in the same way as for other PDSCH data, except that the identity indicated on the PDCCH is not the identity of a specific UE but a designated broadcast identity known as the System Information Radio Network Temporary Identifier (SI-RNTI), which is known a priori to all UE's. SI normally changes only at specific radio frames whose System Frame Number (SFN) is given by SFN mod N=0, where N is configurable and defines the period between two radio frames at which a change may occur, known as the modification period. Prior to performing a change of the system information, the Radio Access Network (called in LTE Enhanced or Evolved UMTS Radio Access Network, E-UTRAN) notifies the UE's by means of a Paging message including a SystemInfoModification flag. The LTE mechanism for indicating to a UE in idle mode that SI has changed is sending a paging message including the SystemInfoModification flag indicating whether or not SI has changed.
In order to receive paging messages from the LTE network, UE's in idle mode monitor the PDCCH for a P-RNTI or a SI-RNTI value used to indicate paging. The UE only needs to monitor the PDCCH channel at certain UE-specific subframes within specific radio frames. At other times, the UE may apply Discontinuous Reception (DRX), meaning that it can switch off its receiver to save battery energy. DRX functionality can be configured for an idle mode UE so that it does not always need to monitor the downlink channels. A DRX cycle consists of an On Duration during which the UE should monitor the PDCCH and a period during which a UE can skip reception of downlink channels.
The E-UTRAN configures which of the radio frames and subframes are used for paging. Each cell broadcasts a default paging cycle. In addition, upper layers may use dedicated signalling to configure a UE-specific paging cycle. If both are configured, the UE applies the lowest value. The UE calculates the radio frame (the Paging Frame (PF)) and the subframe within that PF (the Paging Occasion (PO)), which E-UTRAN uses to page the UE, by means of a procedure which takes into account the UE International Mobile Subscriber Identity (IMSI) and a T parameter, which is determined by the shortest of the UE-specific DRX cycle, if allocated by upper layers, and a default DRX cycle broadcast in system information. If UE specific DRX is not configured by upper layers, the default value is applied (the exact procedure is described in 3GPP TS 36.304 “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode”).
The maximum length of the paging cycle, when the UE can receive a paging indication in a Paging Frame, is equal to 256 frames (that is, 2.56 seconds), which means that the maximum DRX period when the UE can remain switched off (without “listening” the signals from the base station) is 2.56 seconds. During the period when the radiofrequency section of the UE remains switched off, the local oscillator and the timing functions of the UE must remain operative, in order to keep the timing and to know when the DRX period has elapsed and it is necessary to completely switch on the UE for receiving paging messages. The maximum time the UE can remain switched off without resynchronizing its oscillator to the eNB frame depends on the characteristics of the frame and the frequency accuracy of the oscillator.
As stated before the LTE frame is 10 ms long, it is composed of 20 slots of 0.5 ms, each slot with 6 or 7 OFDM symbols, and the first symbol of every slot is preceded by a cyclic prefix that is between 5.2 us and 16.7 us long. Therefore, the maximum acceptable timing error for the UE oscillator after a DRX period is 5.2 us, because if the time error were longer than the cyclic prefix there would be inter-symbol interference when decoding the first symbol. A timing error of 5.2 us in a DRX period of 2.56 s is equivalent to a time error of 2 parts per million (ppm). This means that the UE oscillator frequency has drifted 2 ppm in 2.56 s.
The oscillator in the UE is usually a digitally controlled Voltage Controlled Crystal Oscillator (VCXO), and its frequency drift with time depends on a number of factors, and the most important of them in a time scale of seconds or minutes is a change in its temperature. According to several researches performed in the prior art, to provide an indication of the expected frequency change of a crystal oscillator due to a change of temperature depending on the type of crystal cut and the angle of the cut, the frequency change for a 10° C. variation around the specified temperature of operation, typically 25° C., is below 10 ppm. This means that a simple VCXO installed in a UE should suffer temperature variations of the order of some degrees Celsius in time scales of seconds (which is unlikely) to experience a frequency drift of the order of some parts per million. Hence, in normal operation conditions, when the device temperature will be stable, the VCXO in the UE will keep a frequency accuracy better than 2 ppm for time periods even much longer than 2.56 seconds (in other words, in normal conditions the UE can be switched off without resynchronizing its oscillator to the eNB much more than 2.56 seconds. On the other hand, the maximum time the UE can remain unsynchronized from the LTE frame depends greatly of the type of crystal oscillator in the UE, its current temperature and other UE implementation factors, and therefore only the UE can estimate that time.
The LTE network can request a UE to provide its capabilities using the “UE capability transfer” procedure (as described in 3GPP TS 36.331 “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification” version 11.2.0). The E-UTRAN can indicate for each Radio Access Technology (RAT), for example LTE, whether it wants to receive the associated UE capabilities. The UE provides the requested capabilities using a separate container for each RAT. If the UE-CapabilityRequest message from the LTE network to the UE includes the E-UTRA indicator, the UE will include the UE-EUTRA-Capability Information Element (IE) within a UE-CapabilityRAT-Container and with the RAT-Type set to EUTRA. The encoding of the UE capabilities in the UE-EUTRA-Capability IE, includes among other data the “UE-Category” can range from 1 to 8. Document 3GPP TS 36.306 “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio access capabilities”. Section 4.1 UE-Category, describes the radio access capability parameters of the UE, including the UE-category and the parameters that are not dependent of the UE-category, which can be transmitted from the UE to the LTE network by means of a UE-EUTRA-Capability IE.
However, the information about UE capabilities reported by the UE to the LTE network does not include said estimation of the maximum time the UE can remain unsynchronized from the LTE frame. Actually, the LTE standard does not implement any method for reporting the maximum time a mobile terminal can remain unsynchronized from the LTE frame but being its local oscillator still time aligned with the LTE frame within a given time accuracy. Therefore, if the LTE network implemented discontinuous reception periods much longer than 2.56 seconds, some of the terminals would lose the time alignment and would require a complete resynchronization with the LTE frame, decoding the synchronization signals and probably losing the expected paging messages.
Some Machine Type Communications usage scenarios supported by wireless networks require the operation of the wireless terminal under very low power conditions, in order to enable a battery-powered wireless terminal to work for months or years without replacing its battery. The standard solution in the Wireless Sensors Network technical field is to implement very low working cycles, meaning that the terminal is most of the time switched off and switches on for very short time periods. So in these scenarios where a battery-powered mobile terminal should work for months or years without replacing its battery, discontinuous reception periods much longer than 2.56 seconds are required (of the order of minutes or even hours), current LTE wireless standard cannot therefore be used, as the maximum time for discontinuous operation in current LTE wireless networks is 2.56 seconds.
Hence, it is necessary to extend the time the mobile terminal can be switched off (especially in Machine Type Communications usage scenarios) by implementing mechanisms which allow to report the maximum time each terminal can remain unsynchronized from the LTE frame without losing time alignment in order to allow to extent the discontinuous reception periods and to minimize the time the terminal is on for receiving paging information from the LTE network. None of these mechanisms are implemented in the current LTE standard.
The proposed embodiments of the invention stated below will provide said mechanisms, overcoming at least some of the drawbacks of the prior art solutions.